Cryogenic Guidelines

8/13/2019 Cryogenic Guidelines

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Table of Contents

1.0 Introduction

2.0 Refrigeration ..

2.1 Refrigeration Selection .

2.2 Refrigeration Capacity Determination

2.3 Estimating Thermal Load Due to Conduction

2.4 Estimating Thermal Load Due to Radiation

2.5 Estimating Thermal Load Due to Convection

2.6 Refrigerator Load Curve .

3.0 Dewar Chamber Construction

3.1 Circular End Plates .

.2 Seals .

3.3 0-rings .

3.4 Vacuum Grease

3.5 Roughing Valves ........

3.6 Charcoal Adsorber Traps

3.7 Charcoal Adsorber Construction and Installation

3.8 Materials for Dewar Construction

3.9 Materials for Radiation Shield

3.10 Vacuum Windows

4.0 Flex Lines

5.0 Helium Line Fittings

6.0 Compressor Selection and Maintenance

7.0 Cleaning Procedures - Vacuum Dewar

7.1 Cleaning Procedures - Refrigerator/Compressors

References

Appendix 1

Appendix 2

Table 1, Mean Time Between Failures and Refrigeration

Table 2, Design Chart for 0-ring Face Seal Glands .

Table 3, Compressor CapacitiesTable 4, Suggested Dewar Components

Figure 1, Balzer Refrigerator Load Map

Figure 2, Thermal Conductivity of Type 347 Stainless Steel

Figure 3, Cylindrical Shell Collapsing Pressure Correction Factors

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1.0 Introduction

The first elements in the signal path of a radio telescope contribute the

greatest amount to the system noise temperature, and for this reason radio

astronomy receivers are generally operated at cryogenic temperatures. In

order to reach cryogenic temperatures, a vacuum chamber (Dewar) containing the

receiver is evacuated to a very high vacuum, and a closed-cycle refrigerator

is used to remove the heat. Thus, the cryogenic system is a crucial element

governing a receiver's performance. When designing cryogenic systems, the

designer is faced with conflicting requirements which require careful analysis

to achieve optimum results.

Among the most important considerations are the performance of the

refrigerator, i.e., temperature reached and maintained, and the degree of

vacuum achieved. Because vacuum levels are improved with the condensation of

gases at the lower temperatures, and the convection loading is reduced with

better vacuum, the vacuum and the refrigeration are interdependent and neither

can be compromised. The cryogenic systems at NRAO often operate in the

transition realm between ultra-high vacuum,


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aid the designer in quantifying design parameters, improving the reliability

and functionality by standardization, and documenting proper maintenance and

cleaning procedures of cryogenic systems.

2.0 Refrigeration

In the past, at Green Bank, most of the cryogenically-cooled receivers were

used for relatively short terms, i.e., over periods of a few days to several

weeks. Except for those receivers at the interferometer, the receivers were

not required to operate continuously for periods over one year. However, with

the advent of the GBT, receivers will be expected to run continuously for

periods limited only by the mean time between failure (MTBF) of the

refrigerators or for routine scheduled maintenance. Documentation from the

manufacturer, the VLA site, and records here at Green Bank, show the MTBF for

the different refrigerators to be as shown in Table 1. It is expected that a

receiver remain cold for the length of the MTBF associated with the

refrigerator being used, or for the normal scheduled maintenance period, which

is typically one year.

TABLE 1 .

Mean Time Between Failures and Refrigeration Capacity

REFRIGERATORMTBF CAPACITY, WATTS

MANUFACTURER VLA GREEN BANK 1ST STAGE 2ND STAGE

MODEL 22 CTI 18,400 28 ,100 6.5 W @70K 0.8 W @15K

MODEL 350 CTI 123,650 N/A 16 W @70K 2.5 W @15K

MODEL 1020 CTI 110,750 N/A 38 W @70K 7 W g15K

UCH-130 LEYBOLD N/A 105 W @70K 11 W @151. VLA Memo, March 1996, "MTTF Report on VLA and VLBA Cryogenic Refrigerators"2. VLBA Memo 89, "CTI Model 22 Test Chronology"

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2.1 Refrigerator Selection

Closed-cycle cryogenic refrigeration systems have been used at NRAO since the

late 1960's. There are several companies manufacturing cryogenic

refrigerators, but NRAO has purchased systems from only three of them:

Cryogenic Technology Inc. (CTI), Waltham, NA; Leybold Cryogenic North America

(formerly Balzers), Hudson, NH; and APD Cryogenics, Allentown, PA. Most of

the refrigerators purchased in the past have been from CTI. However, more

recently the Green Bank and Tucson sites have made purchases from Leybold

Cryogenics. In general, the systems purchased from these two companies have

performed reliably, with the exception of CTI's Model 22. On the other hand,

the third company, APD Cryogenics, supplied refri g erators to the VLA many

years ago; these proved to be unreliable and were replaced with CTI

refrigerators. A representative from APD visited Green Bank recently and said

that the problem they had with their systems was due to the compressors and

that it had been corrected. The CTI Model 22 has proven to have unacceptable

reliability and is difficult to maintain. Although the MTBF is approximately

8400 hours, the variance is quite large among refrigerators. The device was

originally developed for short-term use and not for the way we have been

trying to use it.

All of the refrigerators used at NRAO, both past and present, have operated on

the Gifford-McMahon principle. Research is going on to develop better

systems, e.g., the pulse refrigerator which is supposed to minimize the number

of moving parts and increase the reliability. Currently Gifford-McMahon seems

to be the most practical for radiometer use However, designers of cooled

receivers should try to keep abreast of the newer systems and use them when

they become practical.

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2.2 Refrigerator Capacity Determination

To determine the capacity of the refrigerator required, the system designer

must estimate both the conductive and radiation loading to be absorbed by the

refrigerator cold head at both the first and second stages. A third heat load

is due to the heat conducted from the dewar walls to the cold head by the

residual gas inside the dewar (convection). This third component of heat load

is negligible when the vacuum quality is good; therefore, it is not considered

when estimating the total thermal load. However, if the vacuum quality is

allowed to deteriorate, the load due to the convection mode of heat transfer

may become the dominant component. Hence, the importance of obtaining high

vacuum quality should be stressed during the design and operation of the

system. Finally, any power dissipation due to active devices should be added.

2.3 Estimating Thermal Load Due to Conduction

Since the thermal conductivity of materials is in general variable with the

temperature, to obtain an accurate estimate of heat transfer, the integral of

the material's conductivity K(T)needs to be obtained and applied as shown in

the following equation:

A t2H= J k dt ( 1)

where: A = cross section area of the conducting element, cm?L = the conducting elements length, cmk = the thermal conductivity, watts - cm' k"2

- 71 = the colder temperature, K

= the warmer temperature, K

Since the integral of k between T 1 and T 2 is the area under the thermal

conductivity curve bounded by the T 1 and T 2 , the designer can simply estimate

this area graphically. The thermal conductivity curves for different

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3 0OKK dt = 8.80 10 2 mw/ cm

7 OK

0.785 c m [ 8 808.80 10 2 mw/ cm] 69.1 mpt,

10 cmdt

materials used in receiver design can be found in the NBS document, Thermal

Conductivity of Solids at Roam Temperature and Below, or other handbooks on

cryogenics such as Cryogenic Engineering, R. B. Scott.

There are also computer programs available at NRAO which can be used for the

more common materials and are probably the easiest way to estimate the

conductive loading, providing the material in question is available in the

program.

A program which calculates the heat transfer equation, along with other

thermodynamic relations, is located at sadira/s/swhite/cryo/prgrams. However,

the program was written by Ray Sarwinski in BASIC and only selection "A" will

run. (Caution: Program used polynomial fit to K vs T curves, which gives gross

errors for small temperature ranges.) To calculate heat transfer through

solids:

1. Run gwbasic.exe2. Load "crogeni.cs.cs"3. Run

4. Select letter A

EXAMPLE: Determine the conductive heat transfer through a G-10 fiberglass rod

whose length is 10 cm and whose 0.D. = 1 cm and is connected at one end to a

70 K refrigerator station. The other end is attached to the 300 K dewar wall.

T2 dt=

A = 1 1 r2 = 0.785 cm 2

L=10cm

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NOTE: See Appendix 1 for Table of values over common temperature intervals for

common materials. See Appendix 2 for example of thermal load calculation due

to conduction by graphical estimation of thermal conductivity.

2.4 Estimating Thermal Load Due to Radiation

The net exchange of radiant energy between two surfaces is determined by the

geometry of the two surfaces, their temperatures, and their emissivities at

their respective temperatures. An exact value is difficult to obtain because

of the geometry of most systems; however, the following equation is useful for

making rough estimates of the radiation loading when sizing the refrigerator.

4G A

l ( T

2

4 - )

wattsQ-1 A1 1

-+- ( - 1 )C l A 2 e 2

( 2 )

where: Q = radiation heat transfer, wattsA l = area of inner surface, ft2A 2 = area of outer surface, ft 2T, = temperature of inner surface, K

= temperature of outer surface, K e l = emissivity of the inner surface

e 2 = emissivity of the outer surface0 = 0.533 x 10 -8 watts/ft 2 - K Values of emissivity can be found in handbooks such asCryogenic Engineering, R.B. Scott.

EXAMPLE: Determine the heat transferred by radiation from the walls of a

stainless steel cylindrical dewar to a nickel-plated copper radiation shield,

using equation 2, where the dimensions of the two devices are:

Stainless Steel Dewar: ID= 50.0 cm. X 50.0 cm. longNickel-plated copper radiation shield: ID=45.0 cm. X 45.0 cm. long

Determine A l (radiation shield surface area): 7854.0 cm2Determine A 2 (dewar inner surface area): 6361.7 cm2

T 2 =300.0 K T 1 =50.0 K

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From Table on Emissivity of various materials:

E 1 =0.03 for nickel-plated copperE2 =0.08 for polished stainless steel

Substituting these values into equation 2 yields Q= 6.8 watts.

2.5 Estimating Thermal Load Due to Convection

Normally, the heat transfer by convection is negligible after the cold head

has reached temperatures low enough to cause the gases within the dewar to

condense (cryopumping). Typically, a good dewar will cryopump to a level of

10 -7 to 10' torr, and at this pressure negligible heat is transferred by the

residual gas. However, after many months of operation, the cryogenic surfaces

will become coated with frost, the ability to cryopump will be diminished and

the pressure will rise. The frost build up also increases the emissivity of

the cold surface, thereby increasing the load due to radiation heat transfer.

Also, hydrogen, helium, and neon, whose vapor pressures are relatively high at

15 K, will build up if the adsorber trap, whose function is to adsorb these

gases, becomes saturated. When the pressure increases to 10 - 5 torr, the heat

transfer due to convection will become significant. The temperature of the

cold head will rise, causing more outgassing, and the pressure will rise even

higher. This process will continue until the refrigerator warms up. The

dewar should then be allowed to warm up and evacuated before cool-down is

attempted again. The following equation gives the rate of heat transfer due

to residual gas.

Y4-1 PW

=

2.426 x 10-4

A 1 T 2 ) ( 3 )A y- rAT+ - (1 -

1

A2

with the accommodation coefficientsa -

w

, -Te

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where: W = rate of heat transfer, wattsA l and A 2 = area (c') of inner and outer walls,respectivelyp = pressure, micronsT 1 , T 2 = temperatures, K of inner and outer wall, respectivelyT i = effective temperature of incident moleculesT e = effective temperature of reflected moleculesT w = temperature of wallY = specific heat ratio, (Cp/Cv)M = molecular weight of the residual gas.

By using this equation for typical values used in our systems, it can be shown

that where p ^ .01 microns (10' torr) the heat transfer becomes significant.

2.6 Refrigerator Load Curves

Once the thermal load estimates are made for each stage of the refrigerator,

these values may be plotted on the load curves (see Figure 1) supplied by the

manufacturer. Normally, it is desirable to cool low noise amplifiers to 15 K

or less. Plotting the estimated values on the load charts will show whether

that particular refrigerator has sufficient capacity. Sometimes it is

possible to shift loads between the two stages to achieve an optimum load

distribution to give the desired temperatures. If this still does not provide

enough cooling, a larger refrigerator should be selected. A margin of safety

should also be considered, e.g., select a refrigerator with twice the cooling

capacity, if space and expenditures allow this.

3.0 Dewar Chamber Construction

Mechanical Strength Considerations:

Most dewar chambers are cylindrical, and the walls and end plates must be of

sufficient thickness to withstand a pressure of 1 atmosphere (15 psi). The

thickness of the cylindrical walls may be determined using the following

equation:

Pa

= -k E(L/D) 3 ( 4 )

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n=48

x 2.77x10715

. 1)24

=6.41

where: =a atmospheric pressure, 15 psiP= a correction factor found from Figure 3= is the modulus of elasticity, #/in 2 for the wall material

for stainless steel E = 2.77 x 10 7 #/in2for aluminum E = 1.05 x 10 #/in2

= the dewar diameter= the wall thickness= length

n = the safety factor (a typical value would be 4)

EXAMPLE: Determine the wall thickness of a stainless steel dewar whose

diameter is 24 inches and whose length is 24 inches. A minimum safety factor

of four is required.

1) Assume some standard thickness such as 0.100 i

242) The length-to-width ratio, L/D=---=

24along with aD/ t- 024 1 -240 ..

3) Using the data in step 2, a K factor of 48 is determined using

equation (4), and solving for n yields a result of

n- E(t/D)3Pa

Since n>4, the wall thickness is sufficient. A thinner wall thickness could

be tried until the desired safety factor Is obtained.

3.1 Circular End Plates

To determine the required thickness of the end plates, the following equation

may be used3

Pp- 256 E-

5 (5)4

where: M is 0.3 for metalsE is the modules of elasticity, #/in26 is the deflection at the center of the plate, in.D is the plate diameter, in.t, is the plate thickness, in.Pp is atmospheric pressure, 15 #/in2.

3 (1-M2

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(256) (2.77x10 7 ) ( 0.1)0.234inchest =

3\1

3(1-0.32)244

EXAMPLE: Determine the thickness of a 24-inch diameter stainless steel plate

where the deflection is 0.01 in.

3

t=

3 ( m2) Di4

256 E 5

For aluminum, where E=1.05x10 7 #/in 2 , t 1=0.323 in. or 38% thicker, but weight

is reduced by a factor of 2.93.

3.2 Seals

There are basically two types of seals of concern in the construction of dewar

chambers - metal seals and elastomer seals (o-rings). To minimize the effects

of outgassing and permeation, metal seals should be used when practical. The

most common types are conflat flanges, which are available in 12 different

sizes ranging from 1-1/3 inch O.D. to 13-1/4 inches O.D. However, only the

smaller sizes (up to 2-3/4 inches) are normally used in receiver dewar

construction. This type of seal uses a non-reusable copper gasket. Typical

places where this gasket is used are in mounting (1) roughing valves, (2)

Vacion pumps, and (3) cold cathode ion gauge tubes.

3.3 0-rings

Where o-rings are required, the preferred material for vacuum use is butyl

because of its low outgassing and permeability. Although nitrile (compound

N6740-70) has been used in the past, its outgassing rate is almost 6 times

that of butyl, and its permeability is no better than butyl.

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torr, which is a pressure well above the operational vacuum level of most

dewar chambers (operational vacuum is normally in the 10 to 10' range), and

it is not recommended for cryogenic dewar chambers. If a remote-operated valve

is required, Varian's electromagnetic block valve, P/N L8724301, which

operates to 10' torr, would be a good choice.

Sizes:

When open, a valve should have sufficient conductance to prevent undue

reduction of the rough pumps effective speed. For example, a 1-1/2 inch valve

(conductance = 46 1/sec), with a 2 foot by 1-1/2 inch I.D. hose, will reduce

the pumping speed of the Tribodyne 30/120 from 20 cfm to 14.2 cfm. But if the

valve and vacuum line are reduced to % inch and 3/4 inch respectively, as is

the case in several receivers, the effective pumping speed drops to 1.75 cfm,

increasing the pumping time by a factor of eight.

3.6 Charcoal Adsorber Traps

The typical operational cryogenic temperature range of most receiver dewar

chambers is 12-25 K at the second refrigerator stage, and 50-100 K at the

first stage. All gases in the atmosphere, except helium and hydrogen, become

condensed at these temperatures due to the cryocondensation action of the

refrigerated surfaces. The combined vapor pressures of the condensed gases

and the partial pressures of helium and hydrogen at cool down yield a total

pressure in the range of 10' - 10' torr in a typical cryogenic receiver

dewar. The resultant pressure depends on the pumping speed of the cryogenic

surfaces, the pumping speed of the ion pump (if one is used), the pumping

speed of the charcoal adsorber trap due to the cryosorption mechanism, and the

gas load. Those factors affecting the gas load magnitude are: (1) leaks to

atmosphere, (2) virtual leaks (trapped air in cavities), (3) diffusion (gases

dissolved in materials internal to the dewar that outgas), (4) permeation

(atmospheric gases that travel from outside the dewar to inside the dewar by

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diffusion), (5) vaporization (molecules leaving the surfaces of internal dewar

materials, (6) adsorption (atmospheric gas molecules that adhere to surfaces

of the internal materials), and (7) the quantity of gas remaining in the dewar

chamber after the rough pumping procedure is terminated.

At the normal operating pressures of 10' - 10 - 9 torr, insignificant heat

transfer via conduction through gas occurs between the 300 K dewar walls and

the refrigerated surfaces. However, since hydrogen and helium do not

condense, and even though they are very small constituents of the atmosphere,

with time, the partial pressure of these gases, along with the relatively high

vapor pressure of neon, can cause vacuum deterioration to the point that the

heat transfer by residual gas becomes a significant heat load on the

refrigerator. This happens at pressures >10 - torr. How fast this pressure

increase takes place depends on those factors mentioned above which determine

the gas load.

Installing a charcoal trap on the 15 K second stage cryogenic surface reduces

the number of free hydrogen and helium molecules. The activated charcoal,

which is made from coconut shells heated to about 750 C, absorbs large

quantities of hydrogen, helium, neon and other gases when cooled to

temperatures near 15 K by a mechanism known as 'cryosorption". Naturally, the

more charcoal used, the longer cryosorption occurs. In most cases a trap

whose charcoal surface area is about 50 square inches (a plate 5x5 inches,

covered on both sides) is adequate for a year of cryosorption. The activated

charcoal, Union Carbide JXC 6/8 Mesh, which was originally installed, is no

longer manufactured. Calgon Carbon Corporation, X Trusorb 700, is currently

available.

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3.7 Charcoal Adsorber Construction and Installation

The adsorber panel geometry can have any configuration compatible with the

other components in the dewar chamber. However, it should have adequate

surface area so that at least 50 square inches of charcoal is available. When

space is limited, the adsorber could be constructed similar to a finned heat

sink. It is recommended that activated charcoal whose size is approximately

1/8-1/4 inch be bonded to 1/16 inch thick OHFC copper plate, cleaned for high

vacuum use, with Torr Seal epoxy, which is specified to perform to 10 torr.

To improve the bond between the charcoal, Torr seal, and the copper plate, the

plate can be perforated with 1/32 inch diameter holes. The epoxy may be cured

by heating to 60 C for two hours. Prior to bonding the charcoal to the

copper plate, it is recommended that it be dried by heating in a vacuum oven

overnight at a temperature of 400 C.

After the charcoal adsorber is constructed, it should be stored by wrapping in

oiless aluminum foil until it is ready to be installed. Prior to installation,

it is recommended it be baked at 120 C (the max temperature for cured Torr

seal) overnight, and then immediately installed in the vacuum dewar. The time

between installation and vacuum chamber evacuation should be kept to a minimum

to keep the adsorber from becoming contaminated with water vapor from the

atmosphere.

To facilitate maintenance of the adsorber trap, it is recommended that a

thermostatically-controlled heater be installed on the copper plate to allow a

low temperature bake-out be made whenever the dewar chamber requires

evacuation. It is also suggested that a stainless steel tube be installed

from the purging valve to a point close to the adsorber trap so that warm, dry

nitrogen may be sprayed on the charcoal to help rid the charcoal of water

vapor. The warm, dry nitrogen will also help remove water vapor adsorbed to

other internal dewar surfaces. The nitrogen is warmed by a gas purge heater,

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obtained from CTI, which is installed in the nitrogen supply line.

It is also suggested that the trap be installed for easy removal, and that an

identical trap be constructed for replacement when needed. This would allow

the replacement of the traps with the spare that could be baked to 120 C. At

this temperature, the trap would function more efficiently than one baked at

the low temperature provided by the heater.

3.8 Materials for Dewar Construction

One of the most common metals used in vacuum use is stainless steel 304 (S/S-

304). At pressures of 10' torr and lower, S/S-304 is widely used because it

does not oxidize and can be heated to very high temperatures for bake-out to

reduce the component of the gas load caused by diffusion (gases within the

crystalline structure of the metal). Another reason for using S/S-304 is that

it is easily electropolished, which provides a clean surface free of oxidation

and contamination. Electropolishing minimizes the effective surface area and,

in turn, the amount of gas captured on the surface by adsorption. Stainless

steel is also easily welded with the (TIG) Tungsten Inert Gas (argon) method

that is needed for producing vacuum tight welds for high and ultra-high vacuum

operation.

One of the drawbacks of stainless steel is its weight. Where weight is of

major concern, aluminum (whose specific gravity is 2.7, compared to stainless

steel, whose specific gravity is 7.9), might be considered. Although the

modules of elasticity of steel and aluminum are 2.77x10 7 #/in and 1.05x107

#in 3 , respectively, the extra thickness required for strength with aluminum is

only 38% over what is required for stainless steel, but stainless steel weighs

2.9 times more than aluminum, allowing the weight to be at least cut in half.

However, making vacuum tight welds with aluminum can be difficult, and

additional thickness may be required to make the welded seams vacuum tight,

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with the result that the anticipated amount of weight reduction may not be

achieved. Furthermore, aluminum is easily scratched and more prone to leaks

at o-ring gland surfaces. If aluminum is chosen for the dewar chamber

material, considerations might be given to having its internal surface

polished and then electroplated with nickel to maintain a surface that won't

oxidize and is easy to clean. The emissivity of nickel is constant (about 4%

at 300 K); whereas, that for aluminum can vary from 3% to 75%, depending on

the amount of oxide on the surface.

3.9 Materials for Radiation Shields

The function of the radiation shield (usually made of aluminum or copper) is

to minimize the loading effects of the thermal radiation from 300 K dewar

walls on the 15 K cryogenic surfaces. This is done by intercepting the

thermal radiation on a thermally conductive enclosure which surrounds the 15 K

cryogenic surface and is connected to the 70 K station. Thus, the radiation

from the 300 K walls is captured and dissipated by the 70 K stage of the

refrigerator, which has a much higher cooling capacity than the 15 K stage,

thereby conserving the cooling capacity for the electronic components.

However, the 15 K surfaces are radiated by thermal energy from the 70 K

radiation shield, but the amount of irradiation is vastly reduced over what it

would receive if there were no radiation shield.

To reduce the amount of radiation absorbed by the 70 K radiation shield and

re-radiated by the shield to the 15 K surfaces, the material used for the

radiation shield should have high conductivity at 70 K and low emissivity.

The typical metals used are aluminum or copper, whose thermal conductivities

at 70 K are approximately 2.5 and 5 watts-cm - 1 -K, respectively. The

emissivities of aluminum and copper can range between .018 to 0.7 for aluminum

and 0.006 to 0.78 for copper, depending on the surface finish and oxide

content. Because of this variability of emissivity with surface condition,

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and as an aid to maintain a clean, nonoxidized and highly reflective surface,

t is suggested that the radiation shield be polished to a surface finish of

8 , 4 2 in. or less and plated with an electroless nickel to a depth of .0005

inches (12 microns).

3.10 Vacuum Windows

The transition from the atmospheric pressure of the waveguide to the vacuum in

the dewar requires a material with low electrical loss, low permeability to

various gases and a low outgassing rate, while having the mechanical

properties to withstand the 1 atmosphere pressure differential. Unfortunately,

no single material possesses all the desired properties over a wide frequency

range. Typically, a thin plastic film, with its low permeability to gases,

is bonded to a low-loss foam material for strength. Mylar and the Hercules

HR500/2S coated polypropylene packing film both have been used successfully

(Electronics Division Internal Report No 292 and Addendum #1). The

polypropylene has a lower permeability to water vapor and comparable strength

to Mylar.

The selection of foam depends upon the frequency range and, thus, the size of

the window. Emerson-Cuming foam, Eccofoam PS 1.04, was tested and displayed

good electrical properties as well as low outgassing rates. However, the foam

was originally manufactured with CFC's, and the manufacturing technique has

been changed, which increased the outgassing properties to unacceptable levels

and has been found to be too lossy at millimeter wavelengths. A replacement

for the Eccofoam is the expanded foam manufactured by Radva Corporation which

is made out of ARCO Dylite beads. The Radva foam has comparable electrical

properties, but the outgassing properties are unacceptable for windows on the

order of tens of centimeters. Dow Corning manufactures a product called

bouyancy foam, which has higher loss than the Radva foam but better outgassing

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properties. Another alternative is the Gortex RA-7957 expanded PTFE, which

has excellent electrical and outgassing properties, but has only been recently

used by the receivers at the 12 meter telescope (report in preparation). This

foam should be considered for more applications as further results become

available.

NOTE: Reference provides outgassing Data for various materials.

4.0 Flex Lines

Typically, compressors are located some distance from the refrigerators. The

helium supply and return lines experience stresses from movement of the

telescopes. The stresses are primarily from flexing, but sometimes twisting

of the lines occurs. After several cycles of flexing and twisting, the jacket

experiences fatigue and begins to leak.

From experiences with the 300-ft and 140-ft Telescopes, bronze type flex lines

have proven to be superior to stainless steel where stresses are high. The

traveling-feed receiver of the 300-ft flexed the lines over an approximate 3

inch bend radius, causing the stainless steel lines, which have a minimum bend

radius specification of 8 inches, to last only one month. These lines were

replaced with bronze lines, which have a 6 inch minimum bend radius

specification, and lasted an average of one year. Experience with lines in

the tail bearing of the 140-ft has dictated the use of bronze lines for longer

life. Therefore, when selecting flex lines, the amount of stresses due to

flexing and twisting must be considered.

5.0 Helium Line Fittings

In the past all helium lines, either rigid or flexible, were fitted with

Aeroquip self-sealing fittings. These were good for disconnection, but

occasionally leaked, especially in very cold weather. We now use a totally

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stainless steel compression fittings manufactured by Swagelok, which has

proven to be very leak tight even at very cold temperatures. This type of

fitting does not maintain pressure in the line when disconnected; therefore,

they are only used on rigid lines where failures are extremely rare.

continue to use the Aeroquip fittings on flex lines that might possibly break,

allowing the lines to be changed quickly without the loss of helium line

pressure.

6 .0 Compressor Selection

Presently, at Green Bank, two types of compressors supply pressurized helium

to the cryogenic refrigerators. The older compressors are reciprocating, using

piston and valve assemblies purchased from CTI and subsequently modified due

to overheating problems. The piston type compressors are being replaced by

rotary scroll compressors with either a 2.5 HP or 5 HP rating. The type and

number of refrigerators operated from a particular compressor can be derived

from helium mass flow rates of the compressors given in Table 3. The values

for the CTI refrigerators are estimated since they will not divulge this

information.

TA B L E 3

Compressor Type Rating Flow @ Pin

Hitachi 250RHH 2.5 HP 2 5 scfm @ 84 psig

Hitachi 500RHH 5.0 HP 52 scfm @ 84 psig

CTI Piston 1020 (modified) 3.0 HP 4 4 scfm @ 84 psig

Compressor Capacities for use with CTI 1020 (35 scfm), 350 (15 scfm), 22(7scfm) and the Leybold UCH-130 (52 scfm) refrigerators.

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7.0 Cleaning Procedures - Vacuum Dewar

Proper cleaning of a vacuum dewar is the most critical step in having a good

vacuum as opposed to having a great vacuum.

To clean a dewar properly takes several steps, each done methodically and

thoroughly.

Step 1.

A dawar received from the machine shop is generally covered with cutting

fluids. Therefore, it needs to be degreased first to remove these fluids,

which may or may not be oil based. A good degreaser is tap water and any

strong commercial detergent. If visible signs of contaminants remain, a

solvent degreaser should be used.

Step 2.

After removing the outer layer of oil or other cutting fluids, the dewar needs

to have the inner layers of contaminate removed. This is done best with a

product called Citranox, a scouring pad, and a lot of scrubbing. After

scrubbing, rinse with very hot tap water and follow with de-ionized water.

Citranox is sufficient for systems with vacuums approaching 1V-9 Torr.

Step 3.

A final rinse with methanol will complete the cleaning procedure by removing

the majority of surface water.

At this point care must be taken to prevent the dewar from becoming

contaminated before it is assembled. If it is to be assembled immediately, no

extra steps are needed; but if it will be a while before assembly, the dewar

should be stored in an oven, clean work bench, or wrapped in oil-free aluminum

foil.

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Gloves should be worn to protect the hands during cleaning and to protect the

dewar during assembly.

7.1 Cleaning Procedures - Refrigerators/Compressors

Refrigerators and compressor parts are cleaned in a semi-clean environment.

Petroleum ether effectively cleans grease laden components such as the

bearings. A citrus cleaner, ADL enhanced for example, removes most

contaminants from the displacers. The procedure is similar to that used in

cleaning dewars.

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References:

[1] Childs, G. E., et. al., "Thermal Conductivity of Solids at RoomTemperature and Below," NBS Monograph 131, 1973.

Powell, R. L., and Blanpied, W. A., "Thermal Conductivity of Metals and

alloys at Low Temperatures," NBS circular 556, September 1, 1954.

[2] Scott, R. B., Cryogenic Engineering, Van Nostrad, 1959.[3] Ibid, p. 152

[4] Ibid, p. 146

[5] Kerr, A. R., et. als., "A Study of Materials for a Broadband Millimeter-

Wave Quasi-Optical Vacuum Window," Electronics Division Internal Report

No. 292, and MMA Memo No. 90, August 21, 1992, and Addendum #1 to both

documents.

[6] Campbell, William A.,Jr., and Scialdone, John J., "Outgassing Data forSelecting Spacecraft Material," NASA Reference Publication 1124, Rev. 3,

September 1993.

[7] A report on vacuum windows using GoreTex RA-7957 is in preparation.

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Appendix 2

The following is an example showing how the thermal load due to conduction of

a cryogenic component may be estimated, providing the component's thermalconductivity vs. temperature and its dimensions are known.

Determine the conductive load due to heat transfer from a 300 K heat sink to a

10 K cryogenic station by a type 347 stainless steel rod whose 0.D.=1.0 cm,

and whose length is 10 cm.

Rod OD = 1 .0 cm

Hea t

300 K

10 K

10 cm

1) Use graph H in Cryogenic Engineering, p.345, for type 347 stainless steel.

2) Since graph is presented in log form, re-plot in linear form as shown in

Figure 1 using AUTOCAD.

3) Measure the area under the curve from 10 K to 300 K using the AUTOCAD

command "area". In this case, the area measures 31164 mw/cm.

4) Estimated heat transfer is then

H=

A [ area under curve]

A = nR 2 , where R = 0.5 cm, L =10 cm

A= 0.785

0.785cm2H [31164 mW/ cm] = 2.446 x 10 3 mW

10 cm

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H o = Mean O.D. of 0-ring(see Table A6-1)

Tolerance = Minus 1% of Mean0.D., but not more than-.060

FOR EXTERNAL PRESSURE

(inward pressure direction)dimension the groove by itsinside diameter (Hi) and width:

H i = Mean I.D. of 0-ring(see Table A5-1)Tolerance = Plus 1% of Mean

I D., but not more than+.060.

BREAK CORNERSAPPROX. .005 RAD

0 TO 5*(TYP)

L GROOVESURFACE FINISH X: I DEPTH32 FOR LIQUIDS (= G LAND DEPTH )16 FOR VACUUMAND GASES

GLAND DETAIL(Refer to design chart A5-2 below)

H SECTION W-W

i- T -

.003 MAX

-1 I- 005

0-RINGSIZE

PARKER NO. 2

CROSS SECTION

NOMINAL ACTUAL

00 4through 1/16 .070

05 0 003

102through 3/32 .103

178 - .003

201through 1/8 .139

284 .004

30 9through 3/16 .210

395

42 5through 1/4 .275

47 5 .006

Special 3/8 .375+.007

Special 1/2 .500008

*0 preferred

face seal glands

FOR INTERNAL PRESSURE

(outward pressure direction)dimension the groove by itsoutside diameter (Ho) and width:

DESIGN CHART A5-2

FOR 0-RING FACE SEAL GLANDSThese dimensions are intended primarily for face type seals and low temperature applications.

SQUEEZE GROOVE WIDTHGLANDDEPTH

ACTUAL % LIQUIDS ANDVACUUM

GASES

GROOVERADIUS

.050to

.013to

1 9to

.101to

.084to

.005to

.054 .023 32 .107 .089 .015

.074 .020 20 .136 .120 .005to to to to to to

.080 .032 30 .142 .125 .015

.101 .028 2Q .177 .158 .010to to to to to to

.107 .042 30 .187 .164 .025.152 .043 2 1 .270 .239 .020to to to to to to

.162 .063 30 .290 .244 .035

.201 .058 2 1 .342 .309 .020to to to to to to

.211 .080 2 9 .362 .314 .035

.276 .08 2 2 2 .475 .419 .030to to to to to to

.286 .108 28 .485 .424 .045

.370 . 112 2 2 .638 .560 .030to to to to to to

.380 .138 27 .645 .565 .045

A5-13

TABLE 2

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TA B L E 4

' F U N C T I O N COMPANY PART NUMBER I

Purging NUPRO SS-4H

Vac-Ion Valve Varian L6591-307

Vacuum Pump Vacoa FD-ILS-62

DC Feed Thru Detronics DTIH-16-23

SMA Feed Thru Omni Spectra 2084-8001-90

Suggested Dewar Components

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T E MP E R A T U R E K

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Cryogenic Guidelines

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